Where can I download Bitcoin?

First of all, whatever bitcoin software you decide to download, don't forget to backup your wallet! (For more tips on keeping your bitcoins safe, see bitcoinsafety.com)

The best place to download the original bitcoin client software is from bitcoin.org. However this client software is not necessarily the best thing for people who are new to Bitcoin as it can take a long time to set up and can take up many of your computer's resources. At bitcoin.org you will find a few different alternatives for using bitcoin directly.

Online Wallets

Instead of downloading a bitcoin client to your own computer, an alternative is to use an online wallet such as blockchain.info. One issue with using an online wallet is that in some cases, you may need to trust the owner of the site not to make off with your bitcoins. This is where blockchain.info's wallet is different to most other online providers, in that the wallet is encrypted and decrypted with javascript right in your browser so that not even the owners of the website can get access to your coins.
When you log in, it downloads the encrypted wallet onto your computer where it is decrypted by using your password.

Other bitcoin clients that may be worth downloading are: Electrum, Multi-bit, Armory just to name a few. These are all linked to from bitcoin.org.

Further Info

For further information and help with using bitcoin, check out this bitcoin FAQ.